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The basics of device circuit breakers | Influencing factors in the application

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Influencing factors in the application

Some applications often require very long cable paths , which can significantly impair system operation . In the event of a fault , the required tripping current may be limited by overly high line impedance . This results in a shutdown occurring too late . Not infrequently , this can lead to a voltage dip and system downtime . The maximum cable lengths that can used between the power supply and the end device depend on a range of factors :
• Type of power supply ( characteristic curve )
• Power supply maximum current ( Power Boost , SFB Technology , etc .)
• Internal resistance or voltage drop for the protective device being used
• Cable resistance ( material , cross section , length )
• Theoretical contact resistance at terminal points
• Effects of temperature
The factors that can be directly influenced include the type , the cross section , and the length of the cable . The selected cabling path should be as short as possible , since the cable resistance fundamentally counteracts any shortcircuit current . In the case of miniature circuit breakers , a multiple of the nominal current is required to ensure safe and quick shutdown and should be , depending on the characteristic used , up to 15 times the nominal current . With such a high current demand in the event of a fault , the cable resistance very quickly acts as a limiting factor that causes the shortcircuit current not to be detected as such . The result is that shutdown occurs too late and entails a voltage dip for all loads on the same voltage source ( Fig . 18 ).
For that reason , it is of the utmost importance to find the right characteristic curve . It should of course allow the load to start , but also trip promptly in the event of a fault . In this
Maximum short-circuit currents ( 24 V DC )
Distance between power supply / load
case , a characteristic curve like the SFB characteristic , which falls between a B and C characteristic , can often prove useful . However , the power supply should have the required tripping current in reserve . This also serves to ensure
I _
A
15 x l N 12 x l N 10 x l N 6 x l N 4 x l N 2 x l N
1 x l N
l N
0
Conductor cross section
0.34 mm2 0.5 mm2 0.75 mm2 1.0 mm2 1.5 mm2 2.5 mm2 2 m 119 A 175 A 263 A 351 A 526 A 877 A 4 m 60 A 88 A 132 A 175 A 263 A 439 A 6 m 40 A 58 A 88 A 117 A 175 A 292 A 10 m 24 A 35 A 53 A 70 A 105 A 175 A 20 m 12 A 18 A 26 A 35 A 53 A 88 A 30 m 8 A 12 A 18 A 23 A 35 A 58 A 40 m 6 A 9 A 13 A 18 A 26 A 44 A 50 m 5 A 7 A 11 A 14 A 21 A 35 A
Fig . 18 : The table shows the maximum current that can flow through a given copper conductor . This clearly shows how the current is greatly attenuated by the cable resistance . This means that a short circuit may not be able to be shut down promptly under some circumstances .
safe operation . If the cable cannot be run in shorter lengths or larger cross sections , an additional electronic circuit breaker can generally be used as a protective device . This is because an electronic circuit breaker responds to
M1 SFB
F1
Electronic
Fig . 19 : The tripping current for electronic circuit breakers is significantly lower than for thermomagnetic circuit breakers .
PhoENix CoNtACt 19